This invention relates generally to respiratory physiology and deals more particularly with a method and apparatus for monitoring apnea.
In the field of respiratory physiology, it has long been known that post-operative patients and patients with brain damage and other debilitating injuries are susceptible to the sudden cessation of breathing which is known as apnea. Sudden infant death syndrome is caused by apnea and is triggered by an immaturity in the central nervous system that ordinarily corrects itself by the first birthday. Apneic episodes nearly always occur during sleep. Infants that are likely to be subject to sudden infant death syndrome are presently identified by considering a number of factors such as family history, premature birth, and the age and physical condition of the mother. In the intensive care nursery, likely apnea candidates are carefully monitored, and subsequent close monitoring in the home is usually recommended if any signs of susceptibility to apnea are observed in the nursery. It is not uncommon for an infant to first demonstrate apnea at about two months of age.
The various types of apnea monitors that have been proposed are essentially respiration monitors. Among their basic components, they include a transducer which senses respiration and provides a signal, an electronic circuit for conditioning the signal, and an alarm device which provides an alarm following an adjustable time delay after loss of signal. The alarm generates a noise that sometimes awakens the infant such that spontaneous resumption of breathing occurs. If the infant is not awakened, resuscitation is necessary.
Apnea monitors differ primarily in the type of transducer employed. Some monitors employ direct techniques which sense the flow of air in the airway. For active babies up to several months old, direct transducers of this type are impractical. The other type of transducer is an indirect type which detects the physical movement accompanying respiratory effort rather than ventilation itself. Indirect transducers can be either contact types or noncontact types.
Direct contact transducers require that electrodes or other sensing elements be attached directly to the torso or chest of the infant. This encumbers free movement and can cause skin irritation and related problems. Particularly with large infants such as those monitored in the home, it is common for the electrodes to fall off or be pulled off and for the infant to become entangled in the wires leading from the electrodes. Among the types of noncontact sensors that have been proposed are segmented pneumatic mattresses equipped with an anemometer for sensing the flow of air among the segments, mattresses filled with a conductive elastomer, Doppler ultrasound transceivers, and capacitance type transducers that are either incorporated into a rigid mattress or placed under the mattress. All of these noncontact transducers suffer from numerous problems that have detracted from their commercial success. Typically, they are small and/or rigid in order to focus on a limited area so that stability and reproductability can be achieved. However, this introduces problems relating to changes of position of the infant within the crib or bassinet.
Perhaps the most important problem associated with all existing apnea monitors is a lack of reliability. All known monitors are subject to false alarms when apnea does not occur and, even more importantly, they sometimes fail to give an alarm during apnea. Both contact and noncontact transducers are overly sensitive to the beating of the heart and movement of the great vessels, and both types of instruments sometimes interpret this "cardiovascular artifact" as respiration when there is actually an absense of respiration. As a result, the alarm is not given even though apnea has occurred.
Another serious problem with existing apnea monitors involves the manner of displaying whether there is normal breathing or an alarm condition and whether or not the instrument is functioning properly. Oscilloscope displays have been attempted but are so large and expensive as to be virtually prohibitive under most circumstances.
Since the incoming signal varies over a wide range, the gain during apnea should not be allowed to rise to a level that invites problems with artifact. If apnea should occur and the infant is awakened by the resulting alarm, there is no indication given by most existing instruments that there has been an apnea episode. The episode thus passes unnoticed by the parents or nurse.
In existing apnea monitoring instruments, the time delay of the alarm is commonly generated by using a timed zero-crossing technique. If the incoming signal is lost, the timer begins to run but is reset to zero if the signal should reappear before elapse of the time delay period which is typically about 15 seconds. If the infant should twitch during apnea, which is not uncommon, the timer is reset and the alarm is delayed accordingly. Thus, the time delay prior to giving an alarm can be extended such that the alarm is not sounded in sufficient time to either awaken the baby or alert the parents or nurse to the apnea episode.